T lymphocytes are critical mediators of immunity; however they are continuously lost for a variety of reasons throughout life, and therefore must be replaced. Generation of new T cells is the function of the thymus, and the unique stromal microenvironment in the thymus directs T cell development and the selection of self- tolerant, self-restricted T cell population. Unfortunately, the thymus undergoes a precipitous age-induced atrophy resulting in reduced nave T cell production. This reduction triggers an expansion of existing T cells in the periphery, and a shift towards an oligoclonal pool dominated by memory T cells, leaving the elderly population less responsive to new infections and vaccines. Preventing and/or reversing thymic atrophy therefore hold significant potential for healthspan extension. The mechanisms governing age-induced thymic atrophy has been difficult to resolve because stromal cells, which represent the primary targets of atrophy, are rare and difficult to isolate. W have shown that in addition to contracting during aging, stromal cells are also functionally impaired, including a reduction in mechanisms that induce tolerance to self. Importantly, this impairment is not corrected by regenerating the size of the organ experimentally, even though the regenerated organ appears superficially normal. Thus, preservation of thymic function appears to require preventing the damage that leads to accelerated atrophy, rather than simple restoration of size. Our preliminary data strongly suggest that stromal atrophy is profoundly impacted by deficiency in the enzyme catalase (Cat), which is responsible for terminal detoxification of reactive oxygen species (ROS) produced during normal aerobic metabolism, and that complementation of this deficiency mitigates thymic atrophy. We propose to test whether lifelong complementation of antioxidant activity can prolong essential thymic stromal functions and nave T cell function, as well as preventing atrophy. Since a potential outcome of these studies is therapeutic, we also propose performing experiments to determine why ROS may be useful in stromal cells; numerous physiological functions for ROS have been demonstrated, and the fact that stromal cells specifically repress Cat activity suggests an essential requirement for ROS. We will test the hypothesis that ROS-induced modifications to DNA are essential for induction of tissue restricted antigen (TRA) gene expression in stromal cells, and thus for the induction of central tolerance. We will also test the relationship between ROS-induced stress and the induction of autophagy, as it relates to self-antigen presentation and tolerance. Together, the proposed studies aim to comprehensively assess the efficacy and feasibility of using antioxidant activity to prolong immune function with age, while simultaneously uncovering novel mechanistic aspects of thymic biology and pathology.